The substrates for the generation of kinins by kallikrein are high (HMWK) and low molecular weight kininogen (LMWK) in humans and all animals examined except for the rat. The rat has a third trypsin activatible kininogen which generates T-kinin (Ile-Ser-Bradykinin). The pharmacological effects of T-kinin in relation to and combined with Bradykinin (BK) have been investigated in whole animal and bioassay preparation. Preliminary results indicate that T-kinin is biologically active but that the effects are not simply additive to BK but rather that T-kinin modifies the effects of BK. This gives rise to a new hypothesis in which simultaneously generated similar peptides (kinins) can modulate the effects of each other, with a physiological stimulus/effect coupling that would partially depend on the different metabolism of the peptides involved.